Structured surface articles containing geometric structures with compound faces and methods for making same

ABSTRACT

A prepared substrate including an array of cube corner cavities and protrusions interspersed between the cube corner cavities is described herein.

RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.12/177,278, filed Jul. 22, 2008, now U.S. Pat. No. 7,712,904; which is adivision of U.S. patent application Ser. No. 11/931,744, filed Oct. 31,2007, which issued as U.S. Pat. No. 7,562,991; which is a continuationof U.S. patent application Ser. No. 11/728,549, filed Mar. 26, 2007,which issued as U.S. Pat. No. 7,384,161; which is a division of U.S.patent application Ser. No. 10/336,287, filed Jan. 3, 2003, which issuedas U.S. Pat. No. 7,261,425; which is a division of U.S. patentapplication Ser. No. 09/515,120, filed Feb. 25, 2000, which issued asU.S. Pat. No. 6,540,367; which is a continuation-in-part of, and claimspriority under 35 USC Sections 120 and 365(c) to, PCT Patent ApplicationNo. PCT/US99/07618, filed Apr. 7, 1999.

TECHNICAL FIELD

The present disclosure relates generally to structured surfacesfabricated using microreplication techniques. These surfaces haveparticular application to structured surfaces that compriseretroreflective cube corner elements.

BACKGROUND

The reader is directed to the glossary at the end of the specificationfor guidance on the meaning of certain terms used herein.

It is known to use microreplicated structured surfaces in a variety ofend use applications such as retroreflective sheeting, mechanicalfasteners, and abrasive products. Although the description that followsfocuses on the field of retroreflection, it will be apparent that thedisclosed methods and articles can equally well be applied to otherfields that make use of microreplicated structured surfaces.

Cube corner retroreflective sheeting typically comprises a thintransparent layer having a substantially planar front surface and a rearstructured surface comprising a plurality of geometric structures, someor all of which include three reflective faces configured as a cubecorner element.

Cube corner retroreflective sheeting is commonly produced by firstmanufacturing a master mold that has a structured surface, suchstructured surface corresponding either to the desired cube cornerelement geometry in the finished sheeting or to a negative (inverted)copy thereof, depending upon whether the finished sheeting is to havecube corner pyramids or cube corner cavities (or both). The mold is thenreplicated using any suitable technique such as conventional nickelelectroplating to produce tooling for forming cube cornerretroreflective sheeting by processes such as embossing, extruding, orcast-and-curing. U.S. Pat. No. 5,156,863 (Pricone et al.) provides anillustrative overview of a process for forming tooling used in themanufacture of cube corner retroreflective sheeting. Known methods formanufacturing the master mold include pin-bundling techniques, laminatetechniques, and direct machining techniques. Each of these techniqueshas its own benefits and limitations.

In pin bundling techniques, a plurality of pins, each having a geometricshape such as a cube corner element on one end, are assembled togetherto form a master mold. U.S. Pat. No. 1,591,572 (Stimson) and U.S. Pat.No. 3,926,402 (Heenan) provide illustrative examples. Pin bundlingoffers the ability to manufacture a wide variety of cube cornergeometries in a single mold, because each pin is individually machined.However, such techniques are impractical for making small cube cornerelements (e.g., those having a cube height less than about 1 millimeter)because of the large number of pins and the diminishing size thereofrequired to be precisely machined and then arranged in a bundle to formthe mold.

In laminate techniques, a plurality of plate-like structures known aslaminae, each lamina having geometric shapes formed on one end, areassembled to form a master mold. Laminate techniques are generally lesslabor intensive than pin bundling techniques, because the number ofparts to be separately machined is considerably smaller, for a givensize mold and cube corner element. However, design flexibility suffersrelative to that achievable by pin bundling. Illustrative examples oflaminate techniques can be found in U.S. Pat. No. 4,095,773 (Lindner);International Publication No. WO 97/04939 (Mimura et al.); and U.S.application Ser. No. 08/886,074, “Cube Corner Sheeting Mold and Methodof Making the Same”, filed Jul. 2, 1997.

In direct machining techniques, series of groove side surfaces areformed in the plane of a planar substrate to form a master mold. In onewell known embodiment, three sets of parallel grooves intersect eachother at 60 degree included angles to form an array of cube cornerelements, each having an equilateral base triangle (see U.S. Pat. No.3,712,706 (Stamm)). In another embodiment, two sets of grooves intersecteach other at an angle greater than 60 degrees and a third set ofgrooves intersects each of the other two sets at an angle less than 60degrees to form an array of canted cube corner element matched pairs(see U.S. Pat. No. 4,588,258 (Hoopman)). Direct machining techniquesoffer the ability to accurately machine very small cube corner elementsin a manner more difficult to achieve using pin bundling or laminatetechniques because of the latter techniques' reliance on constituentparts that can move or shift relative to each other, and that mayseparate from each other, whether during construction of the mold or atother times. Further, direct machining techniques produce large areastructured surfaces that generally have higher uniformity and fidelitythan those made by pin bundling or laminate techniques, since, in directmachining, a large number of individual faces are typically formed in acontinuous motion of the cutting tool, and such individual facesmaintain their alignment throughout the mold fabrication procedure.

However, a significant drawback to direct machining techniques has beenreduced design flexibility in the types of cube corner geometries thatcan be produced. By way of example, the maximum theoretical total lightreturn of the cube corner elements depicted in the Stamm patentreferenced above is approximately 67%. Since the issuance of thatpatent, structures and techniques have been disclosed which greatlyexpand the variety of cube corner designs available to the designerusing direct machining See, for example, U.S. Pat. No. 4,775,219(Appledorn et al.); U.S. Pat. No. 4,895,428 (Nelson et al.); U.S. Pat.No. 5,600,484 (Benson et al.); U.S. Pat. No. 5,696,627 (Benson et al.);and U.S. Pat. No. 5,734,501 (Smith). Some of the cube corner designsdisclosed in these later references can exhibit effective aperturevalues well above 67% at certain observation and entrance geometries.

Nevertheless, an entire class of cube corner elements, referred toherein as “preferred geometry” or “PG” cube corner elements, have upuntil now remained out of reach of known direct machining techniques. Asubstrate incorporating one type of PG cube corner element is shown inthe top plan view of FIG. 1. The cube corner elements shown there eachhave three square faces, and a hexagonal outline in plan view. One ofthe PG cube corner elements is highlighted in bold outline for ease ofidentification. The highlighted cube corner element can be seen to be aPG cube corner element because it has a non-dihedral edge (any one ofthe six edges that have been highlighted in bold) that is inclinedrelative to the plane of the structured surface, and such edge isparallel to adjacent nondihedral edges of neighboring cube cornerelements (each such edge highlighted in bold is not only parallel to butis contiguous with nondihedral edges of its six neighboring cube cornerelements). Disclosed herein are methods for making geometric structures,such as PG cube corner elements, that make use of direct machiningtechniques. Also disclosed are articles manufactured according to suchmethods, such articles characterized by having at least one speciallyconfigured compound face.

BRIEF SUMMARY

Structured surface articles such as molds or sheetings are disclosed inwhich a geometric structure has a plurality of faces. At least one ofthe faces is a compound face comprising a machined portion and anon-machined portion. The non-machined portion can be formed by, forexample, replication from another substrate or embossing with a suitabletool. A transition line may separate the machined portion from thenon-machined portion. The geometric structure can of course comprisefaces arranged to form a cube corner element.

Cube corner elements, and structured surfaces incorporating an array ofsuch elements, are disclosed wherein at least one face of the cubecorner element terminates at a nondihedral edge of such element, theface comprising two constituent faces disposed on opposed sides of atransition line that is nonparallel to the nondihedral edge. The cubecorner element can comprise a PG cube corner element, and exactly one,exactly two, or all three faces of such element can comprise twoconstituent faces disposed on opposed sides of a transition line that isnonparallel to the respective nondihedral edge. In an array ofneighboring cube corner elements, each cube corner element in the arraycan have at least one face configured as described above. Further, thecube corner elements can be made very small (well under 1 mm cubeheight) due to the direct machining techniques employed.

Molds are disclosed in which the structured surface comprises pyramidsdisposed proximate to at least one cavity, the cavity being formed bynon-machined faces and the pyramids being formed at least in part bymachined faces.

Methods are disclosed for making a structured surface article comprisingat least one geometric structure. The method includes providing aprepared substrate having a non-machined face, and removing materialfrom the prepared substrate to form a machined face such that themachined face and the non-machined face together form one of the facesof the geometric structure.

Methods are also disclosed for forming in a substrate a structuredsurface that extends along a reference plane and contains PG cube cornerelements. The method includes providing a prepared substrate, andforming groove side surfaces in the prepared substrate that extend alongaxes that are substantially parallel to the reference plane. Facesformed by the groove side surfaces together with other facesincorporated in the prepared substrate combine to form the PG cubecorner elements.

Various methods are disclosed for providing the prepared substrate. Onesuch method begins by forming an array of non-machinable protrusions (orcavities) in a first initial substrate, whether by mechanical, chemical,electromagnetic, or other suitable means. A negative copy of the firstinitial substrate is made in a second initial substrate composed of amaterial suitable for machining. Upper portions of the protrusions inthe second initial substrate are machined to form pyramids. A negativecopy of the second initial substrate is then made to form the preparedsubstrate. The prepared substrate includes cavities corresponding to thepyramids formed in the second initial substrate, and also includesprotrusions between the cavities. In some embodiments, the pyramids arecube corner pyramids and the cavities are thus cube corner cavities.

With such a prepared substrate, groove side surfaces are then formed byselectively machining the protrusions in such a way that the machinedfaces formed in the protrusions are in substantial alignment with faces(“non-machined faces”) of neighboring cube corner cavities, which faceshad been replicated from the second initial substrate. At least one faceof the PG cube corner elements is a compound face that includes both oneof the machined faces and one of the non-machined faces. The compoundface may include a transition line that separates the replicated facefrom the machined face. Retroreflective sheeting or other cube cornerarticles can then be replicated from the prepared substrate as modifiedby the direct machining operations.

Another method for providing the prepared substrate begins with theprepared substrate having a substantially flat working surface. An arrayof cube corner cavities is then formed deep in the working surface byembossing with a hardened tool. Intermediate portions of the workingsurface forming protrusions between the cube corner cavities are leftunfinished. Groove side surfaces are then formed selectively in theprotrusions to form machined faces, the machined faces and the embossedfaces of the cube corner cavities together forming the array of cubecorner elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a structured surface comprising one type of PGcube corner element array, known from the PRIOR ART;

FIG. 2 is a perspective view of a prepared substrate;

FIG. 3 is a perspective view of the substrate of FIG. 2 after machiningsome groove side surfaces;

FIG. 4 is a perspective view of the substrate of FIG. 2 after machiningall of the groove side surfaces;

FIG. 5 is a top plan view of FIG. 4;

FIGS. 6-8 are top plan views of structured surfaces having canted PGcube corner elements, such surfaces being capable of fabrication usingthe methods discussed in connection with FIGS. 2-5;

FIG. 9 is a perspective view of a masked substrate;

FIG. 10 is a perspective view of the substrate of FIG. 9 after formingprotrusions therein with the aid of the mask;

FIG. 11 is a perspective view of a substrate having a non-machinablestructured surface replicated from the substrate of FIG. 10;

FIG. 12 is a perspective view of the substrate of FIG. 11 after directlymachining groove side surfaces in the protrusions thereof to form cubecorner pyramids therein;

FIG. 13 is a perspective view of a copy of the substrate of FIG. 12,forming a prepared substrate analogous to that of FIG. 2;

FIGS. 14 a-d are schematics showing 2-dimensional representations of theprogression from a structured surface article closely approximating adesired geometry to an article or articles that have the desiredgeometry;

FIGS. 15 a-d depict the tips of different cutting tools that can beemployed in forming groove side surfaces;

FIGS. 16 a-f are schematic sectional views that show in magnifiedfashion the region where two constituent faces of a compound face cometogether, demonstrating different types of possible transition lines;

FIG. 17 is a plan view of a PG cube corner cavity in which eachtransition line between constituent faces has a finite width;

FIG. 18 a is a plan view of an initial substrate having an array offour-sided protrusions, and FIG. 18 b is a sectional view thereof asindicated;

FIG. 19 is a plan view of the substrate of FIGS. 18 a-b after forminggroove side surfaces in the upper portions of the protrusions;

FIG. 20 a is a plan view of a substrate obtainable by making a negativecopy of the substrate of FIG. 19 and machining groove side surfaces inupper portions of such negative copy;

FIG. 20 b is a sectional view as indicated in FIG. 20 a;

FIG. 21 is a perspective view of a group of laminae;

FIG. 22 is an endwise elevational view of the laminae in a tiltedposition and having a set of grooves machined in the working surfacesthereof;

FIG. 23 is a side elevational view of the laminae of FIG. 22;

FIG. 24 is a magnified endwise elevational view of the laminae aftertilting them back into alignment and machining their working surfacesfurther;

FIG. 25 is a plan view of a structured surface of a prepared substrate,the structured surface being a negative copy of the structured surfaceformed by the laminae of FIG. 24;

FIG. 26 is the substrate of FIG. 25 after forming groove side surfacestherein;

FIG. 27 is a top plan view of an initial substrate after forming grooveside surfaces in upper portions of protrusions;

FIG. 28 is a plan view of a substrate obtainable by making a negativecopy of the substrate of FIG. 27 and machining groove side surfaces inupper portions of such negative copy;

FIG. 29 is a plan view of the substrate of FIG. 28 after machining anadditional set of parallel grooves therein; and

FIGS. 30-32 show an alternative embodiment in an analogous fashion toFIGS. 27-29.

In the drawings, the same reference symbol is used for convenience toindicate elements that are the same or that perform the same or asimilar function.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

In FIG. 2, a prepared substrate 10 is shown enlarged in perspectiveview. A description of how prepared substrate 10 can be fabricated isdeferred for later discussion below. Substrate 10 has a structuredsurface 12 that generally defines a reference plane x-y. Structuredsurface 12 includes faces 16 arranged in groups of three that form cubecorner cavities 18. Interspersed between cube corner cavities 18 onstructured surface 12 are protrusions 20. The protrusions as shown eachhave three side surfaces and a top surface, and are of triangular crosssection. Depending on the procedure used to make the prepared substrate,the side surfaces of the protrusions 20 can be inclined to a greater orlesser extent away from the vertical. In the figure, reference points 22located on the top surfaces of protrusions 20 are shown for ease ofdescription. It is not critical that the tops of protrusions 20 passthrough the locations marked by reference points 22, nor is it criticalthat the side surfaces or top surface have a well defined shape,orientation, or surface finish. This is because outer portions ofprotrusions 20 are removed in subsequent direct machining operations.

FIG. 3 depicts the prepared substrate 10 at an intermediate stage duringsuch a direct machining operation. Cutting tools 24 a,24 b,24 c(collectively referred to as 24) move along structured surface 12,whether by motion of the cutting tools or the substrate or both, to formgroove side surfaces 26 a (hidden from view from the perspective of FIG.3), 26 b,26 c respectively. Each tool 24 is depicted as a so-called“half-angle” tool, which produces only one groove side surface as itprogresses through the material rather than a pair of opposed grooveside surfaces, although this is not necessary. Consistent with directmachining procedures, cutting tools 24 move along axes 28 a,28 b,28 cthat are substantially parallel to the x-y reference plane, thusensuring that the respective groove side surfaces also extend along axesthat are substantially parallel to the reference plane. Preferably, theaxes 28 a,28 b,28 c are carefully positioned and the tool orientationcarefully selected so that the groove side surfaces are substantiallyaligned (translationally and rotationally) with adjacent faces 16 of thecube corner cavities 18. Relatively small misalignments, discussedinfra, may however be tolerated or may even be desirable as a functionof the desired optical characteristics of the cube corner article andthe precision of available cutting machinery. Regardless of the degreeof alignment, transition lines 30 separate groove side surfaces fromfaces 16 of cavities 18. As seen in FIG. 3, a protrusion 20 that hasbeen machined by all of the cutting tools forms a protrusion referred toas a geometric structure 20 a, which structure includes one face fromeach groove side surface 26 a,26 b,26 c. In the case where the threefaces of structure 20 a are substantially aligned with adjacent faces 16of cavities 18, and where such cavities 18 have a common orientation,the three faces of structure 20 a (when considered separately) form a“truncated” cube corner pyramid. Such a pyramid is characterized byhaving exactly three nondihedral edges that form a “base triangle” inthe plane of the structured surface.

Substrate 10 is composed of a material that can be scribed, cut, orotherwise machined without significant post-machining deformation andwithout substantial burring. This is to ensure that the machined faces,or replications thereof in other substrates, can function as effectiveoptical reflectors. Further discussion on suitable substrate materialsis given below.

It should be noted that although three cutting tools are shown in FIG.3, a single cutting tool can be used. The cutting tool can be made ofdiamond or other suitably hard material. The machined faces can beformed by any one of a number of known material removal techniques, forexample: milling, where a rotating cutter, spinning about its own axis,is tilted and drawn along the surface of the substrate; fly-cutting,where a cutter such as a diamond is mounted on the periphery of arapidly rotating wheel or similar structure which is then drawn alongthe surface of the substrate; ruling, where a nonrotating cutter such asa diamond is drawn along the surface of the substrate; and grinding,where a rotating wheel with a cutting tip or edge is drawn along thesurface of the substrate. Of these, preferred methods are those offly-cutting and ruling. It is not critical during the machiningoperation whether the cutting tool, the substrate, or both aretranslated relative to the surroundings. Full-angle cutting tools arepreferred when possible over half-angle tools because the former areless prone to breakage and allow higher machining rates. Finally,cutting tools having a curved portion or portions can be used in thedisclosed embodiments to provide non-flat (curved) surfaces or faces inorder to achieve desired optical or mechanical effects.

FIG. 4 is a view of substrate 10 after all groove side surfaces havebeen formed by direct machining operations. As shown, all of theprotrusions 20 previously seen in FIGS. 2 and 3 have been modified toform geometric structures 20 a. Each geometric structure 20 a comprisesthree machined faces, one each from groove side surfaces 26 a, 26 b, and26 c, configured approximately mutually perpendicular to each other toform a truncated cube corner pyramid. Each of these three faces ismachined to be substantially aligned with the nearest face of anadjacent cube corner cavity 18. Because of this, new cube cornercavities 32 are formed, each new cube corner cavity 32 comprising onecube corner cavity 18 and one face each of its neighboring geometricstructures 20 a. Reference numeral 32 a shows in bold outline one suchcube corner cavity 32. A given face of one of the cube corner cavities32 comprises one face of a cube corner cavity 18 and one of the machinedfaces 26 a, 26 b, or 26 c. As will be discussed infra, faces 16 of thecube corner cavity 18 are non-machined faces. Therefore, each cubecorner cavity 32 comprises a compound face made up of a non-machinedportion and a machined portion. The transition lines 30 separate themachined from the non-machined portions.

One can also identify new cube corner pyramids 34 formed on thestructured surface shown in FIG. 4. Each cube corner pyramid 34comprises one geometric structure 20 a, which is a cube corner pyramid,and one face each of its neighboring cube corner cavities 18. Each faceof one of the pyramids 34 is a compound face comprising a non-machinedface 16 from one of the cavities 18 and a machined face from structure20 a. Reference numeral 34 a shows in bold outline one such cube cornerpyramid 34. Note that the reference points 22 locate the uppermostextremities or peaks of the pyramids 34. Both cube corner pyramids 34and cube corner cavities 32 are PG cube corner elements, since both havea face terminating at a nondihedral edge of the cube corner element,such nondihedral edge being nonparallel to reference plane x-y.

FIG. 5 shows a top view of the structured surface of FIG. 4. Transitionlines 30 are drawn narrower than other lines to aid in identifying thePG cube corner elements, i.e., cube corner cavities 32 and cube cornerpyramids 34. The compound faces of such PG cube corner elements have amachined and non-machined portion disposed on opposed sides oftransition lines 30. In the embodiment of FIGS. 2-5, all transitionlines 30 lie in a common plane referred to as a transition plane, whichin the case of this embodiment is coplanar with the x-y plane. Thenon-machined faces of the structured surface are disposed on one side ofthe transition plane and the machined faces are disposed on the otherside.

Although structural differences between machined and non-machinedsurfaces are subtle, such differences can generally be detected byinspection of the underlying material. Some suitable inspectiontechniques include examination of the grain or crystalline structure,molecular orientation, or variations in the amount of trace elementssuch as chromates or lubricants.

The directly machined cube corner article of FIGS. 2-5 can itselffunction as a retroreflective article, both with respect to lightincident from above (by virtue of cube corner cavities 32) and, wherethe substrate is at least partially transparent, with respect to lightincident from below (by virtue of cube corner pyramids 34). In eithercase, depending upon the composition of the substrate, a specularlyreflective thin coating such as aluminum, silver, or gold can be appliedto the structured surface to enhance the reflectivity of the compoundfaces. In the case where light is incident from below, reflectivecoatings can be avoided in favor of an air interface that provides totalinternal reflection.

More commonly, however, the directly machined prepared substrate ofFIGS. 2-5 is used as a mold from which end-use retroreflective articlesare made, whether directly or through multiple generations of molds,using conventional replication techniques. Each mold or other articlemade from the directly machined prepared substrate will neverthelesscontain cube corner elements having at least one face terminating at anondihedral edge of the cube corner element, the at least one facecomprising two constituent faces disposed on opposed sides of atransition line, the transition line being nonparallel to suchnondihedral edge. As seen from FIGS. 4 and 5, transition lines 30 lie inthe transition plane coincident with the x-y plane, whereas nondihedraledges shown in bold for both PG cube corner cavity 32 and PG cube cornerpyramid 34 are inclined relative to the x-y plane. It is also possibleto fabricate surfaces where the transition lines do not all lie in thesame plane, by forming groove side surfaces at different depths in thesubstrate.

A wide variety of structured surfaces can be fabricated using the directmachining technique described above. The PG cube corner elements of FIG.5 each have a symmetry axis that is perpendicular to the x-y referenceplane of the structured surface. Cube corner elements typically exhibitthe highest optical efficiency in response to light incident on theelement roughly along the symmetry axis. The amount of lightretroreflected by a cube corner element generally drops as the incidenceangle deviates from the symmetry axis. FIG. 6 shows a top plan view of astructured surface 36 similar to that of FIG. 5, extending along the x-yplane, except that the PG cube corner elements of FIG. 6 are all cantedsuch that their symmetry axes are tilted with respect to the normal ofthe structured surface. The symmetry axis for each PG cube corner cavity46 in FIG. 6 lies in a plane parallel to the y-z plane, having avertical component in the +z direction (out of the page) and atransverse component in the +y direction. Symmetry axes for the PG cubecorner pyramids 48 of FIG. 6 point in the opposite direction, withcomponents in the −z and −y directions. In fabricating surface 36, aprepared substrate (see FIG. 13, infra) is used wherein the protrusionsof generally triangular cross-section are isosceles in shape, ratherthan equilateral as in FIG. 2. Further, the non-machined cube cornercavity faces are arranged to have a similarly shaped isosceles triangleoutline.

Four distinct types of cube corner elements are present on thestructured surface 36: truncated cube corner cavities havingnon-machined faces and a triangular outline in plan view; truncated cubecorner pyramids having machined faces and triangular outline; PG cubecorner cavities having compound faces and a hexagonal outline; and PGcube corner pyramids, also having compound faces and a hexagonaloutline. A representative non-machined cube corner cavity is identifiedin FIG. 6 by bold outline 40, and a representative machined cube cornerpyramid is identified by bold outline 42. Transition lines 44 separatemachined from non-machined faces, and all such lines 44 lie in atransition plane parallel to the x-y plane. In other embodiments, thetransition lines may lie parallel to a transition plane but not becoplanar. Selected faces of cavities 40 and pyramids 42 form canted PGcube corner elements, in particular canted PG cube corner cavities 46and canted PG cube corner pyramids 48. Reference points 22, as before,identify localized tips or peaks disposed above the x-y plane.

FIG. 7 shows a structured surface 36 a similar to that of FIG. 6, andlike features bear the same reference numeral as in FIG. 6 with theadded suffix “a”. PG cube corner elements of FIG. 7 are canted withrespect to the normal of structured surface 36 a, but in a differentdirection compared to that of the PG cube corner elements of FIG. 6. Thesymmetry axis for each PG cube corner cavity 46 a is disposed in a planeparallel to the y-z plane, and has a vertical component in the +zdirection and a transverse component in the −y direction.

FIG. 8 shows a structured surface similar to that of FIGS. 6 and 7, andlike features bear the same reference numeral as in FIG. 6 with theadded suffix “b”. PG cube corner elements in FIG. 8 are also canted,but, unlike the PG cube corner elements of FIGS. 6 and 7, the degree ofcant is such that the octagonal outline in plan view of each PG cubecorner element has no mirror-image plane of symmetry. The cube cornercavities of FIG. 8 each have a symmetry axis that has components in the+z, +y, and −x direction. It will be noted that the triangles formed bytransition lines 44 (FIG. 6) are isosceles triangles each having onlyone included angle less than 60 degrees; triangles formed by lines 44 a(FIG. 7) are isosceles triangles each having only one included anglegreater than 60 degrees; and triangles formed by lines 44 b (FIG. 8) arescalene triangles. Representative values in degrees for the includedangles of such triangles are, respectively: (70, 70, 40); (80, 50, 50);and (70, 60, 50).

With the aid of FIGS. 9-13, methods for fabricating a prepared substratesuitable for use in the invention will now be described. Forillustrative purposes a structured surface useful for producing thestructured surface of FIG. 6 is described. The same principles canhowever be straightforwardly applied to other embodiments. In brief, astructured surface comprising an array of protrusions is formed in afirst substrate (FIGS. 9-10), by a process other than direct machining Anegative copy of the structured surface is then made in a secondsubstrate (FIG. 11) composed of a substance capable of being machined.Upper portions of protrusions in the structured surface of the secondsubstrate are then directly machined to form cube corner pyramids (FIG.12). Finally, a negative copy of the structured surface of the machinedsecond substrate is made in a third substrate (FIG. 13) to form theprepared substrate, in which an array of cube corner cavities (havingreplicated faces) is intermeshed with an array of protrusions. Themachined second substrate can if desired be used thereafter as a masterfrom which a large number of identical prepared substrates can beelectroformed or otherwise replicated.

Many variations of the procedure are possible. In one, the first andsecond substrates are bypassed, and the desired structured surface isimparted directly into the prepared substrate using an embossing tool.In another, the first substrate is composed of a machinable material sothat cube corner pyramids are formed in the first substrate and thethird substrate is replicated directly from the first substrate ratherthan from an intermediate second substrate. As discussed below,machinable laminae can also be used in manufacturing the preparedsubstrate.

Turning to FIG. 9, a substrate comprising flat lower and upper layers50, 51 is shown carrying a patterned masking layer 52. Masking layer 52is patterned in the form of intermeshing arrays of triangles, the sizeand shape of which are selected to closely approximate the network oftransition lines appearing in the finished mold (see FIG. 6). Triangularwindows 52 a have been formed so that layer 52 comprises an array oftriangular-shaped islands 52 b. One way of forming such a patternedmasking layer 52 is with conventional photoresist and well knownphotolithographic techniques. For example, layer 52 can initially beapplied to the substrate as a uniform layer of metal. Then, a layer ofpositive or negative photoresist is applied on top of layer 52. Portionsof the photoresist are selectively exposed to light using a mask thatbears the triangular array pattern, or its inverse, and subsequentlyexposed or unexposed portions of the photoresist are chemically removed.This opens triangular-shaped windows in the photoresist, so that asecond chemical applied to the surface can selectively attack exposedportions of layer 52. After removing the remaining photoresist, onlytriangular islands of layer 52 remain on the substrate surface.

In FIG. 10, protrusions 56 having the desired cross-sectional shape havebeen formed in the substrate by exposing the masked surface to ananisotropic etching agent and etching completely through upper layer 51.Lower surfaces are shown shaded in FIG. 10 and in subsequent FIGS.11-13. Although it is possible for layers 50 and 51 to be composed ofthe same material, in a more preferred approach upper layer 51 iscomposed of a polymeric material, layers 50 and 52 comprise a metal suchas copper that functions as an etch stop, and the etching agent is anintense electromagnetic beam illuminating the entire upper surface.Exposed regions of layer 51 are ablated by the electromagnetic beam, andregions protected by islands 52 b are left intact. The electromagneticetching agent proceeds through the material substantially only along anaxis approximately normal to the surface, rather than isotropically.This behavior avoids undercutting substrate material beneath maskinglayer islands 52 b, yielding reasonably well-formed cavities with highlysloped walls and substantially flat bottoms, the cavities defining anarray of complementary protrusions 56. After etching completely throughlayer 51 a depth D into the substrate, the anisotropic etching agent isremoved and the remaining masking layer is subsequently eliminated.Depth D is preferably selected to be equal to or greater than the cubeheight of the PG cube corner elements in the finished mold.

In another approach, a substrate similar to that of FIG. 9 is usedexcept that it includes no patterned masking layer 52. Triangularcavities are instead formed by exposing the upper layer 51 to anelectromagnetic beam that is itself patterned by a conventional mask andoptical system such that areas corresponding to 52 a are illuminated andareas corresponding to 52 b are not. An example of this approach wasdemonstrated on a substrate in which lower layer 50 was copper and upperlayer 51 was a 0.125 mm thick piece of Kapton H film sold by E. I. DuPont de Nemours and Company, although Kapton V film and other polyimidesare also useable. The electromagnetic etching agent was a pulsedkrypton-fluoride laser having a wavelength of 248 nm and an energydensity between about 0.5 and 1.2 Joules/cm². Following the ablationprocedure, side walls of the protrusions were found to be sloped atabout a 5 degree angle measured from the normal, inclined in such a waythat the bases of the protrusions were slightly larger than the tops.

Alternative techniques for producing the surface shown in FIG. 10 willbe readily apparent to those skilled in the microfabrication arts,techniques such as ion milling, knurling, chemical etching, and even hotmelting where the substrate is composed of a soft, low melting pointmaterial such as wax. The technique known as LIGA, described forinstance in Chapter 6 of Fundamentals of Microfabrication by Marc Madou,(CRC Press 1997), is another available technique.

In general, the substrate of FIG. 10 might not be composed of a materialsuitable for directly machining smooth, accurate surfaces therein.Therefore, the surface features of FIG. 10 may need to be replicated orotherwise copied in another substrate, shown in FIG. 11 with referencenumeral 58, such substrate 58 composed of a material suitable for directmachining Substrate 58 is shown to have a structured surface that is anegative copy of the structured surface of substrate 50. Thus, cavities60 correspond substantially to protrusions 56, and protrusions 62correspond substantially to cavities in substrate 50 that are bounded byprotrusions 56. A substrate having a positive copy of the structuredsurface of substrate 50 could equally well be used in place of substrate58.

The upper portions of protrusions 62 are then directly machined with asuitable cutting tool such as a full- or half-angle tool. The tools areguided along axes that are parallel to the substrate and preciselyaligned with borders of the triangular protrusions in order to formgroove side surfaces 64 a,64 b,64 c as shown in FIG. 12. The depth ofthe cutting tool is limited to some fraction, preferably about one-half,of the dimension D. The groove side surfaces are cut at angles such thattruncated cube corner pyramids 66 are formed on the upper halves ofprotrusions 62. As a consequence of machining only the upper portions ofprotrusions 62, the lower portions of cavities 60 remain intact, suchlower portions referred to as reduced cavities 60′. The reduced cavitieshave a depth D′, which preferably satisfies the following relationship:D′≧(desired PG cube height)−(cube corner pyramid 66 cube height)

Finally, to obtain the prepared substrate, a negative copy of thestructured surface of substrate 58 is made by a suitable replicationtechnique in a substrate 68, which is the prepared substrate in whichthe PG cube corner elements shown in FIG. 6 are later formed. Substrate68 has a structured surface 70 comprising cube corner cavities 72corresponding to cube corner pyramids 66. Significantly, the faces ofeach cube corner cavity 72 have been formed not by machining surface 70but instead by replicating surface 70 from another structured surface.Therefore, the faces of cube corner cavities 72 are referred to asnon-machined faces. Protrusions 74 are also included on structuredsurface 70, the protrusions corresponding to reduced cavities 60′ andhence having a height D′. Substrate 68 is composed of a machinablematerial so that precision groove side surfaces can later be machined inthe prepared substrate in registration with the non-machined faces ofcavities 72 to yield the desired structured surface geometry.

Regarding canting of cube corner elements, it will be apparent that ifcanted cube corner elements are desired in the final article for a givenoptical effect, the cube corner pyramids 66 will be configured to becanted so that the corresponding cube corner cavities 72 as well as thePG cube corner cavities 46 (see FIG. 6) will likewise be canted.Alternatively the cube corner elements need not be canted.

As was discussed in connection with FIG. 10, alternative techniquescapable of producing a prepared substrate such as shown in FIG. 2 or 13will be readily apparent to those skilled in the microfabrication arts.In one such alternative technique, a single pin of triangularcross-section and having a cube corner pyramid formed on one end thereofcan emboss into a substrate of a suitable plastic material an array ofrecessed cube corner cavities using a step-and-repeat process. If theplastic material is machinable, it can then function as the preparedsubstrate, and in a later operation groove side surfaces can be machinedinto protrusions between the cube corner cavities. In this case theresulting geometric structures will have at least one compound surfacehaving a machined portion and a non-machined portion, the non-machinedportion being an embossed face of the cube corner cavity formed by thepin. If the substrate is not machinable, then a prepared substrate canbe formed by producing a positive copy of the structured surface in sucha machinable material. An even number of conventional replication stepscan be used to produce the positive copy.

A variation of the pin embossing technique just described is where aplurality of pins are held together and simultaneously emboss the cubecorner cavities to a common depth in an initially flat surface.

Prepared substrates can take on a variety of forms. In each embodimentshown above, the prepared substrate has a structured surface comprisingan array of cube corner cavities and protrusions, each protrusion havingsteeply inclined side walls and a large flattened top surface. Whengroove side surfaces are later formed in the protrusions, the cuttingtool removes a relatively large amount of material because the anglebetween the steeply inclined side wall and the subsequent machined faceis often in excess of 10 degrees, typically ranging from about 10 toabout 45 degrees. It may be desirable to provide a modified preparedsubstrate that has a structured surface more closely resembling thefinished mold, for example, a substrate whose individual faces are nomore than a few degrees, and preferably in the range of about 2 to about0.5 degrees, from the desired orientation. Groove side surfaces can thenbe formed in such a modified prepared substrate by removing much lessmaterial from the protrusions during the final groove side surfaceformation, thereby reducing tool forces which could detrimentally causedistortions. Another benefit is less wear on the cutting tool. Amodified prepared substrate can also be used as a master from which afamily of differently configured daughter molds can be made.

FIGS. 14 a-d depict schematically three-dimensional structured surfacesin two dimensions for simplicity. It is to be understood that suchstructured surfaces would commonly comprise geometric structurescomprising at least three-sided pyramids and at least three-sidedcavities. In FIG. 14 a, a modified prepared substrate 76 is providedwith a structured surface 78 comprising actual faces 80 a-d as shown.Transition lines 82 (shown as points), lying in a transition plane 84,separate faces 80 a,b from faces 80 c,d. Reference faces 86 a-d are alsoshown, in broken lines, to represent the desired position of some or allfaces in the structured surface of the final mold, which also terminateat transition lines 82. Where the final mold is intended to beretroreflective, reference faces 86 a,86 d are parallel to each otherand perpendicular to faces 86 b,86 c.

Several features distinguish modified prepared substrate 76 frompreviously described prepared substrates such as substrate 10 of FIG. 2.First, faces 80 c,d are more inclined than faces 86 c,d, yieldingcavities that are slightly deeper than desired. Such deeper cavities canbe formed for example by replicating pyramids that are higher thanordinarily required. Second, protrusions formed by faces 80 a,b likewisedeviate only slightly from the desired configuration. Faces 80 a,b aremore inclined than faces 86 a,b, yielding protrusions that are slightlyhigher than desired. Significantly, the actual faces intersect therespective reference faces in transition plane 84, along transitionlines 82.

The portions of substrate 76 disposed above plane 84 can then bemachined, shaving off the small amount of material necessary to produceprotrusions having the desired faces 86 a, 86 b. More preferably,however, substrate 76 is not itself machined but instead is left intactfor use in making additional molds. In such case a negative copy ofstructured surface 78 is first made in another substrate 90 (FIG. 14 b).Features of substrate 90 that correspond to features of substrate 76 aregiven the same reference number, but with the addition of a primesymbol. Faces 80 c′,80 d′, formed by replication from faces 80 c,80 d,are not shown in FIG. 14 b because a subsequent direct machining stephas formed groove side surfaces in the protrusions so that theyterminate at desired faces 86 c′,86 d′. Note that cavities formed byfaces 80 a′,80 b′ are deeper than desired, because the protrusions fromwhich they were replicated are higher than desired.

Note that the structured surface 92 of substrate 90 contains cavitiesand pyramids comprising compound faces, one compound face made up offaces 86 d′ and 80 a′ and another compound face made up of faces 80 b′and 86 c′. Constituent faces 86 d′ and 80 a′ are only slightlymisaligned with each other, as are constituent faces 80 b′,86 c′. Hence,structured surface 90 has the interesting property that it contains cubecorner cavities (represented by compound face 86 d′-80 a′ and compoundface 80 b′-86 c′) in which the portion of the cavities on one side ofplane 84′ comprise mutually perpendicular constituent faces and theportion on the other side of plane 84′ comprise constituent faces thatare not mutually perpendicular, although they are nearly so. The cubecorner pyramids formed on structured surface 90 can be characterized inthe same way.

Proceeding to FIG. 14 c, a substrate 94 is shown having a structuredsurface 96 that is replicated from structured surface 92. Double primeshave been appended to the reference numerals which otherwise correspondto the designations in FIGS. 14 a and 14 b. As with structured surface92, structured surface 96 contains compound face cube corner elements inwhich the portion of the elements on one side of plane 84″ comprisemutually perpendicular constituent faces and the portion on the otherside of plane 84″ comprise constituent faces that are not mutuallyperpendicular, although they are nearly so. Substrate 94 shown in FIG.14 c can serve as a prepared substrate for the final mold shown in FIG.14 d. Groove side surfaces are directly machined in the substrate by acutting tool that stays on the upper side of plane 84″ and removes smallamounts of material from the protrusions to form machined faces 86 a″and 86 b″, which are in substantial alignment with replicated faces 86d″ and 86 c″ respectively.

The cube corner elements disclosed herein can be individually tailoredso as to distribute light retroreflected by the articles into a desiredpattern or divergence profile, as taught by U.S. Pat. No. 4,775,219(Appledorn et al.). For example, compound faces that make up the PG cubecorner elements can be arranged in a repeating pattern of orientationsthat differ by small amounts, such as a few arc-minutes, from theorientation that would produce mutual orthogonality with the other facesof cube corner element. This can be accomplished by machining grooveside surfaces (both those that ultimately become the faces in thefinished mold below the transition plane as well as those that becomefaces in the finished mold above the transition plane) at angles thatdiffer from those that would produce mutually orthogonal faces by anamount known as a “groove half-angle error”. Typically the groovehalf-angle error introduced will be less than ±20 arc minutes and oftenless than ±5 arc minutes. A series of consecutive parallel groove sidesurfaces can have a repeating pattern of groove half-angle errors suchas abbaabba . . . or abcdabcd . . . , where a, b, c, and d are uniquepositive or negative values. In one embodiment, the pattern of groovehalf-angle errors used to form faces in the finished mold above thetransition plane can be matched up with the groove half-angle errorsused to form faces in the finished mold below the transition plane. Inthis case, the machined and non-machined portions of each compound facewill be substantially angularly aligned with each other. In anotherembodiment, the pattern used to form one set of faces can differ fromthe pattern used to form the other, as where the faces below thetransition plane incorporate a given pattern of nonzero angle errors andfaces above the transition plane incorporate substantially no angleerrors. In this latter case, the machined and non-machined portions ofeach compound face will not be precisely angularly aligned with eachother.

Advantageously, a substrate such as substrate 76 discussed in connectionwith FIG. 14 a can serve as a master substrate from which a whole familyof daughter molds can be made, all having the same general shape of cubecorner element in plan view but having slightly different faceconfigurations. One such daughter mold can incorporate cube cornerelements that each have compound faces whose constituent faces arealigned, the compound faces all being mutually perpendicular to theremaining faces of the cube corner element (see e.g., FIG. 14 d).Another such daughter mold can incorporate cube corner elements thatalso have compound faces whose constituent faces are aligned, but thecompound faces can differ from orthogonality with remaining faces of thecube corner element. Still another such daughter mold can incorporatecube corner elements that have compound faces whose constituent facesare not aligned (see e.g., FIGS. 14 b, 14 c). All such daughter moldscan be made from a single master mold with a minimal amount of materialremoved by machining

Transition Lines

In the preceding figures, transition lines between constituent faces ofa compound face have been illustrated as simple lines or dots.Transition lines can in general take on a great variety of forms,depending upon details of the cutting tool used and on the degree towhich the motion of the cutting tool is precisely aligned with otherfaces in the process of forming groove side surfaces. Although in manyapplications transition lines are an artifact to be minimized, in otherapplications they can be used to advantage to achieve a desired opticalresult such as a partially transparent article.

FIG. 15 shows greatly magnified schematic profiles of some possiblecutting tools useable with disclosed processes. FIGS. 15 a and b depicthalf-angle tools, and FIGS. 15 c and d depict full-angle tools.Flattened tips are provided in the tools of FIGS. 15 a and c.Sharp-edged tips are provided in those of FIGS. 15 b and d. Otherpossibilities include tools having a rounded or radiused tip. Each ofthese tools can comprise a synthetic diamond or other suitable hardmaterial as is known to those of skill in the art.

FIG. 16 shows, also in greatly magnified fashion, schematic sectionalviews depicting the region where two constituent faces of a compoundface come together, demonstrating different types of possible transitionlines. Each view is along the axis of the respective transition line. InFIG. 16 a, a machined face 104 is formed in near perfect registrationwith a non-machined face 106, yielding a nearly imperceptible transitionline 108 a of minimal width. The line may be detectable only byobserving a difference in microscopic surface texture between face 104and 106. In FIG. 16 b, a flat-tipped cutting tool positioned too farinto the substrate material produces a small horizontally-disposed flatsurface making up transition line 108 b. In FIG. 16 c, a cutting toolpositioned too far away from the substrate material leaves a smallvertically-disposed remnant of the protrusion wall to form transitionline 108 c. In FIG. 16 d, a sharp-tipped tool positioned too far intothe substrate material and too deep produces a jagged transition line108 d. In FIG. 16 e, a sharp-tipped tool that is misaligned when forminggroove side surfaces both in the prepared substrate and in a predecessorsubstrate (from which the prepared substrate is replicated) produces aneven more jagged transition line 108 e. Transition line 108 f of FIG. 16f is like line 108 e, except that line 108 f is made using flat-tippedcutting tools.

FIG. 17 shows the effect of having transition line 108 b (see FIG. 16)in place of idealized transition lines 30 in the structured surfaceshown earlier in FIGS. 4 and 5. FIG. 17 shows a top plan view of one ofthe PG cube corner cavities 32. Constituent faces 16 and 26 a, 26 b, 26c are separated by flat transition lines 108 b. Assuming the faces arehighly reflective so that cavity 32 is retroreflective, reflections ofeach transition line will be visible and are shown as 108 b′.(Reflections of the three dihedral edges of PG cube corner cavity 32will also be visible, but are not shown to avoid confusion.) The areataken up by the three lines 108 b as well as their counterparts 108 b′detracts directly from the area that is effective for retroreflection.Thus, if retroreflective active area is to be maximized, the width ofthe transition lines will be minimized by carefully controlling thecutting tool position. It may be desirable however to make an articlethat is not only retroreflective but also behaves like a simple flatmirror, or that is partially transparent. In such cases a transitionline such as 108 b may be used to achieve such results.

Additional Embodiments An Embodiment That Is Not Fully Retroreflective

In FIGS. 18 a and 18 b, a substrate 110 having an array of four-sidedprotrusions and cavities is shown. Protrusions are defined by uppersurfaces 112 and adjoining sloped side surfaces 114, while cavities aredefined by lower surfaces 116 and the adjoining surfaces 114. Depressedflat surfaces are shown shaded in the top plan view of FIG. 18 a, aswell as in FIG. 19. The surface of substrate 110 can be formed in thesame way as that of substrate 58 discussed previously in connection withFIG. 11.

As shown in FIG. 19, groove side surfaces 118, 120 are formed in theupper portions of the protrusions by the action of cutting tools movingalong axes 122, 124 respectively. The groove side surfaces formfour-sided pyramids in the protrusions. The pyramid peaks are identifiedby dots 126. Note that lower surfaces 116 and the lower portions of sidesurfaces 114 are left intact.

A negative copy of the machined substrate 110 is made using standardreplication techniques in a substrate 128. Peaks 126 in substrate 110produce pits 126 a in substrate 128, the pits 126 a lying at the bottomof downwardly inclined faces 118 a, 120 a which correspond respectivelyto upwardly inclined groove side surfaces 118, 120 of substrate 110.Flat-topped protrusions are formed in substrate 128 by lower surfaces116, but are shown in FIGS. 20 a and 20 b after forming groove sidesurfaces 130, 132 therein by action of a cutting tool along axes 134,136 respectively. Groove side surfaces 130, 132 are in substantialalignment with adjacent faces 118 a, 120 a respectively, and formpyramids with pyramid peaks 138. Transition lines 140 separate themachined faces from replicated faces 118 a, 120 a. Adjacent machined andreplicated faces, each of which are three-sided, combine to formfour-sided compound faces. Such compound faces define extended geometricstructures (both cavities centered at pits 126 a and pyramids centeredat peaks 138, see outline 142 of a representative extended cavity) thatare both wider and deeper than either of the simple pyramids formed fromtriangular faces 118, 120 in substrate 110 or from triangular faces 130,132 in substrate 128.

Substrate 128, or positive or negative replicas thereof, can be used fora variety of purposes. If faces 118 a are made mutually perpendicular,and likewise for faces 120 a, 130, and 132, then the article canfunction as a so-called flashing retroreflector which is retroreflectiveonly in selected planes of incidence. Such article is retroreflectiveonly for a light source whose direction of incidence lies in a planeperpendicular to the plane of FIG. 20 a and parallel either to axes 134or to axes 136. Such article can be illuminated either from thestructured surface side or from the flat side opposite the structuredsurface, depending upon the details of construction.

Substrate 128 can also be used as an abrasive surface, or as a mold toproduce abrasive replicas. For such an application, faces 118 a can bemade mutually perpendicular, as can faces 120 a, 130, and 132, or theycan be oriented at smaller or larger angles to yield desired abrasiveproperties.

An Embodiment Making Partial Use of Laminae

In embodiments disclosed above, the direct machining technique is usedtogether with other techniques such as etching, embossing, replicating,and so on to produce structured surfaces never before associated withdirect machining The example that follows shows how it is possible forthe direct machining technique to also be used in conjunction withnon-unitary techniques such as the laminate technique. In brief, aprepared substrate is produced by replicating a structured surfaceformed by a group of laminae whose working surfaces have been machined.The prepared substrate is then directly machined to yield a finishedstructured surface.

In FIG. 21, a plurality of individual laminae 146 each having a workingsurface 148 are held together in a fixture (not shown) that defines abase plane 150. Each lamina is composed of a material suitable formachining smooth burr-free surfaces. The laminae are disposed on andperpendicular to the base plane in FIG. 21. FIG. 22 shows an endelevational view looking down axis 152 after the laminae are tilted orrotated in the fixture about axis 152 by an angle θ, and after a set ofadjacent v-shaped grooves is formed in the working surfaces 148 by acutting tool moving parallel to an axis 154 that is perpendicular toaxis 152 and parallel to plane 150. The grooves have groove sidesurfaces 155, 157 that intersect at upper edges 156 and lower edges 158.FIG. 23 shows a side elevational view of the laminae as viewed along theaxis 154. Adjacent groove side surfaces 155, 157 are approximatelymutually perpendicular.

The laminae are then set upright as in FIG. 21 and an additional grooveside surface 160 is formed in the working surface of each laminae byaction of a cutting tool moving parallel to axis 152. Surface 160 isformed approximately mutually perpendicular to surfaces 155, 157,thereby defining a row of cube corner pyramids in the working surface ofeach laminae, each cube corner pyramid comprising one each of surfaces155, 157, and 160. Finally, every other laminae is offset in a directionparallel to axis 152 such that edges 156 of one lamina line up withedges 158 of its adjacent laminae. A magnified endwise elevational viewof three laminae as just described is shown in FIG. 24, the viewotherwise corresponding to that of FIG. 22.

A negative copy of the structured surface produced by the laminae isthen made in a unitary substrate 162, a top plan view of which is shownin FIG. 25. Faces and edges corresponding to those of FIG. 24 areidentified with the same reference number with the addition of a prime.Since the working surface of the laminae defines cube corner pyramids,the structured surface of substrate 162 has cube corner cavities formedby replicated faces 155′, 157′, and 160′. Both the cube corner pyramidsof FIG. 24 and the cube corner cavities of FIG. 25 are PG cube cornerelements because at least one nondihedral edge (158 or 158′) is inclinedrelative to the plane of the structured surface and is parallel to anadjacent nondihedral edge of a neighboring cube corner element.Substrate 162 can be considered a prepared substrate, with an array ofcube corner cavities interspersed with an array of protrusions 164, onlyone of which is outlined in broken lines in FIG. 25. Each protrusion 164has a triangular base that bounds three triangular faces: an upperportion of face 155′; and upper portion of face 157′; and asubstantially vertical face shown at 166 that joins the other two faces.

Direct machining is then performed on prepared substrate 162 to formgroove side surfaces 168 in protrusions 164, the groove side surfacesbeing in substantial alignment with faces 160′ and extending along axesthat are parallel to the plane of the structured surface. Transitionlines 170 also extend along such axes and separate machined faces 168from replicated faces 160′. Tips or peaks 172 are formed in protrusions164 at the intersection of machined faces 168 and the remaining portionsof replicated faces 155′, 157′. Adjacent faces 168 and 160′ form acompound face.

A comparison of FIG. 26 with FIG. 5 reveals that the structured surfaceof substrate 162 has PG cube corner elements, both PG cube cornerpyramids and PG cube corner cavities, just as substrate 10 does. Alsoapparent is the fact that whereas cube corner elements of substrate 10included three compound faces, each comprising two constituent facesdisposed on opposed sides of a transition line, cube corner elements ofsubstrate 162 have only one compound face. Finally, the structuredsurface of FIG. 26 is seen to comprise an array of three-sided pyramids164 having one machined face 168, and an array of cavities defined bynon-machined faces, the cavities comprising the remainder of thestructured surface (the portions of faces 155′, 157′, 160′ that liebelow the plane defined by transition lines 170), and each pyramiddisposed proximate to and at least partially extending above one of thecavities.

Embodiments Having Optically Opposed Cube Corner Pyramids Without CubeCorner Cavities

Another embodiment is shown by the sequence of FIGS. 27-29. FIG. 27shows a plan view of an initial substrate 180 having a structuredsurface comprising an array of protrusions 182 and cavities 184 (shownshaded), the cavities 184 being bordered by substantially vertical wallsof the protrusions 182. Substrate 180 is similar to the substrate ofFIG. 12, except that the protrusions and cavities of substrate 180 arefour-sided diamond shapes in horizontal cross section rather thantriangles. Groove side surfaces a, b as well as c, d have been formed inthe substrate by the action of cutting tools along axes 186 and 188respectively. Axes 186, 188 are parallel to the plane of the structuredsurface, thus ensuring that side surfaces a, b, c, d all extend alongaxes parallel to such plane. The geometry of the cutting tool isselected to configure surfaces “a” substantially perpendicular tosurfaces c, and to configure surfaces b substantially perpendicular tosurfaces d. Each protrusion 182 thus has four faces a, b, c, d that areinclined relative to the plane of the structured surface and that meetat an elevated peak identified by a dot 189. Note that faces a, b, c, ddo not form a cube corner element.

A negative copy of this structured surface is then, by electroforming orother suitable means, made in a substrate 190 referred to herein as aprepared substrate. Faces a-d of initial substrate 180 form replicatedfaces a′-d′ in prepared substrate 190. Cavities 184 of substrate 180form protrusions 192 in substrate 190, such protrusions shown in FIG. 28after groove side surfaces e, f, g, h have been formed therein bycutting tools moving along axes 194, 196. The cutting tools arecontrolled to form such surfaces substantially parallel to and insubstantial registration with the respective replicated faces so thatpairs of individual faces a′ and f, b′ and e, c′ and h, and d′ and gform compound faces, respectively designated face a′f, face b′e, facec′h, and face d′g. The faces a′f are substantially perpendicular tofaces c′h, and faces b′e are substantially parallel to faces d′g. Dots198 locate the peaks of pyramids formed by faces e, f, g, h. Transitionlines 200, all disposed substantially in a common transition planeparallel to the plane of the figure, separate machined faces e-h fromnon-machined faces a′-d′. FIG. 29 depicts substrate 190 after formingtherein a set of parallel grooves comprising opposed groove sidesurfaces i and j by action of a cutting tool along axes 202 as shown. Inthe embodiment shown, surfaces i and j are inclined at the same anglerelative to the normal to the structured surface, although this is by nomeans required. Such grooves extend deeper into substrate 190 thantransition lines 200, preferably extending to a depth about equal to thelocal minima disposed at the intersection of faces a′, b′, c′, d′. Thecutting tools remove the highest portions of the structured surface,shifting the uppermost peaks from points 198 (FIG. 28) to points 204.

Surface i is configured to be substantially perpendicular to compoundfaces b′e and d′g, thus forming one group of PG cube corner pyramidslabeled 206. Surface j is configured to be substantially perpendicularto compound faces a′f and c′h, forming another group of PG cube cornerpyramids labeled 208. Pyramids 206, 208 are matched pairs of cube cornerelements because one corresponds to a 180 degree rotation of the otherabout an axis perpendicular to the structured surface, and because thereis a one-to-one correspondence of pyramids 206 to pyramids 208. Notethat each pyramid 206, 208 has exactly two faces that are compound. Notealso that the structured surface contains no cube corner cavities.However, truncated non-machined faces a′, b′, c′, d′ do form cavities,and pyramids formed by machined faces e, g, and i, or by machined facesh, f, and j, are arranged on the structured surface such that aplurality of such pyramids border a given cavity.

The sequence of FIGS. 30-32 depict a variation of the embodiment justdescribed. In FIG. 30, an initial substrate 210 has an array ofprotrusions 212 and cavities 214 (shown shaded) therein, with grooveside surfaces a, b, c, d formed in the upper portions of the protrusionsby cutting tools acting along axes 216, 218. Lower portions of theprotrusions have substantially vertical walls, not visible in top planview. Cavities 214 extend to and are bordered by such vertical walls ofa large number of protrusions because such protrusions do not form afully interconnected array in plan view, as in previous figures, butrather only a partially interconnected array. Cavities 214 can thus bereferred to as “open” cavities rather than “closed” cavities, as theyare not substantially entirely bounded by protrusion walls on all sides.As before, surfaces “a” are perpendicular to surfaces c, and surfaces bare perpendicular to surfaces d. Points 220 locate the peaks of thepyramids formed by surfaces a, b, c, d.

FIG. 31 shows a negative copy of the structured surface in anothersubstrate 222. Faces a-d produce replicated faces a′-d′. Cavities 214produce extended protrusions 224, on which have been formed groove sidesurfaces e, f, g, h by action of one or more cutting tools along axes226, 228, the cutting tools controlled to form surfaces e, f, g, hcoplanar with surfaces b′, a′, d′, c′ respectively thus forming compoundfaces a′f, b′e, c′h, and d′g. Transition lines 230 separate machinedfaces e-h from non-machined faces a′-d′. Points 232 locate the peaks ofpyramids formed by these compound faces.

FIG. 32 shows substrate 222 after forming a set of three differentparallel grooves therein along axes 234, 236, 238. The groove along axis236 is formed by a sharp-edged tool such as that of FIG. 15 b or FIG. 15d and comprises opposed groove side surfaces i and j. The grooves alongaxes 234 and 236 are formed with tools having flattened tips such asthat of FIG. 15 a or 15 c. Opposed groove side surfaces k and l inclineupwardly from flat groove bottom m, as do groove side surfaces n and ofrom flat groove bottom p. Surfaces i, k, and n are parallel to eachother and perpendicular to compound faces b′e and d′g. Surfaces j, l,and o are parallel to each other and perpendicular to compound faces a′fand c′h. This geometry results in six different types of PG cube cornerpyramids on the structured surface: pyramids 240, having faces k, b′e,and d′g; pyramids 242, having faces l, a′f, and c′h; pyramids 244,having faces i, b′e, and d′g; pyramids 246, having faces j, a′f, andc′h; pyramids 248, having faces n, b′e, and d′g; and pyramids 250,having faces o, a′f, and c′h. Pyramids 240 and 242 are matched pairs, asare pyramids 244, 246, and pyramids 248, 250. The depth of the final setof parallel grooves is adjusted so that the peaks of the cube cornerpyramids 240, 242, 244, 246, 248, and 250 are all disposed at the sameelevation, although this is not required. Thus, the groove along axis236 has the deepest groove bottom, the groove along axis 234 has theshallowest groove bottom (m), and the groove along axis 238 has a groovebottom (p) of intermediate depth.

The use of grooves having different groove bottoms as shown impacts theoptical performance of retroreflective articles made from the structuredsurface of substrate 222. First, flat groove bottoms m, p produce flatfeatures in the retroreflective article that can act as windows, makingthe article partially transparent. Second, the various groove geometriesaffect the so-called aspect ratio of the cube corner elements, which inturn impacts retroreflective performance as a function of viewing angle.Aspect ratio as used herein relates to the degree of elongation of theoutline of a cube corner element seen in top plan view. For example, thePG cube corner pyramid 244 shown in bold outline has a left and a rightedge that are separated by a given width, and an upper and a lower edgethat are separated by a given length. The ratio of length to width isthe aspect ratio for that particular cube corner element. It can bereadily seen that the PG cube corner pyramids of substrate 222 all havethe same width but that pyramids 244 and 246 have the greatest length,pyramids 240 and 242 have the shortest length, and pyramids 248 and 250have an intermediate length. Adjustment of the aspect ratio of the cubecorner apertures is desirable because it can tailor the divergenceprofile (for a fixed source position, the amount of retroreflected lightas a function of viewing angle) and the entrance angularity (for a fixedviewing angle, the amount of retroreflected light as a function ofsource position) of the cube corner article.

Providing grooves with flat groove bottoms as shown in FIG. 32 have anadditional benefit when such flat groove bottoms are deeper in thesubstrate than the sharp recessed points or edges located at theintersection of other faces. In such case, negative copies of thesubstrate, which can be joined together to scale-up a larger mold, willbe more robust and less prone to damage because the highest features onthe surface of the negative copies will be flat-topped ridges. Placingsuch a negative copy face-down on a flat surface, the flat-topped ridgeswill themselves experience little damage because of their large surfacearea, and further will protect sharp points or ridges formed byneighboring faces from damage due to contact with such flat surface.

Cube corner elements of FIGS. 29 and 32 can be canted or uncanted asdesired. Producing cube corner elements that are canted to a greater orlesser degree is accomplished by tailoring the shape of thediamond-shaped protrusions (FIGS. 27, 30) and then the orientation ofthe groove side surfaces (a, b, c, d, e, f, g, h, i, j, etc.) to be inconformance with the desired degree of canting. It has already beennoted that the techniques used to make the embodiments of FIGS. 29 and32 produce matched pairs of optically opposed cube corner elements. Ifcanting is used, then such matched pairs can, in keeping with principlesdiscussed in U.S. Pat. No. 4,588,258 (Hoopman), U.S. Pat. No. 5,812,315(Smith et al.), and U.S. Pat. No. 5,822,121 (Smith et al.), give rise towidened retroreflective angularity so that an article having thestructured surface will be visible over a widened range of entranceangles.

Turning again to FIGS. 32, the structured surface shown there can beviewed, similar to the surfaces of FIGS. 4, 5, 6, 7, 8, 12, 14 b, 14 d,19, 20 a, 20 b, 26, 27, 28, and 29, 30, 31, as comprising cavitiesformed by non-machined faces and pyramids formed at least in part bymachined faces, each of the pyramids disposed proximate to at least oneof the cavities. The truncated non-machined faces a′, b′, c′, d′ in FIG.32 form the cavities, and the pyramids are formed by machined faces e,g, and one of k, i, or n, or by machined faces h, f, and one of l, j, oro.

Discussion

The working surface of the mold substrates can have any suitablephysical dimensions, with selection criteria including the desired sizeof the final mold surface and the angular and translational precision ofthe machinery used to cut the groove surfaces. The working surface has aminimum transverse dimension that is greater than two cube cornerelements, with each cube corner element having a transverse dimensionand/or cube height preferably in the range of about 25 μm to about 1 mm,and more preferably in the range of about 25 μm to about 0.25 mm. Theworking surface is typically a square several inches on a side, withfour inch (10 cm) sides being standard. Smaller dimensions can be usedto more easily cut grooves in registration with non-machined surfacesover the whole structured surface. The substrate thickness can rangefrom about 0.5 to about 2.5 mm. (The measurements herein are providedfor illustrative purposes only and are not intended to be limiting.) Athin substrate can be mounted on a thicker base to provide rigidity.Multiple finished molds can be combined with each other, e.g., bywelding in known tiling arrangements to yield a large tiled mold thatcan then be used to produce tiled retroreflective products.

In the manufacture of retroreflective articles such as retroreflectivesheeting, the structured surface of the machined substrate is used as amaster mold which can be replicated using electroforming techniques orother conventional replicating technology. The structured surface caninclude substantially identical cube corner elements or can include cubecorner elements of varying sizes, geometries, or orientations. Thestructured surface of the replica, sometimes referred to in the art as a‘stamper’, contains a negative image of the cube corner elements. Thisreplica can be used as a mold for forming a retroreflective article.More commonly, however, a large number of suitable replicas areassembled side-by-side to form a tiled mold large enough to be useful informing tiled retroreflective sheeting. Retroreflective sheeting canthen be manufactured as an integral material, e.g., by embossing apreformed sheet with an array of cube corner elements as described aboveor by casting a fluid material into a mold. See, JP 8-309851 and U.S.Pat. No. 4,601,861 (Pricone). Alternatively, the retroreflectivesheeting can be manufactured as a layered product by casting the cubecorner elements against a preformed film as taught in PCT applicationNo. WO 95/11464 (Benson, Jr. et al.) and U.S. Pat. No. 3,684,348(Rowland) or by laminating a preformed film to preformed cube cornerelements. By way of example, such sheeting can be made using a nickelmold formed by electrolytic deposition of nickel onto a master mold. Theelectroformed mold can be used as a stamper to emboss the pattern of themold onto a polycarbonate film approximately 500 μm thick having anindex of refraction of about 1.59. The mold can be used in a press withthe pressing performed at a temperature of approximately 175° to about200° C.

The various mold substrates discussed above can generally be categorizedinto two groups: replicated substrates, which receive at least part oftheir structured surface by replication from a prior substrate, and bulksubstrates, which do not. The substrate shown in FIG. 9 is an example ofa bulk substrate, as are the laminae 146 of FIGS. 21-24. Replicatedsubstrates can be further categorized into those whose structuredsurface is subsequently machined—such as prepared substrate 10 of FIGS.2-5, substrate 58 of FIGS. 11-12, prepared substrate 68 of FIG. 13,substrate 90 of FIG. 14 b, prepared substrate 94 of FIGS. 14 c-14 d,substrate 110 of FIGS. 18 a-b and 19, substrate 128 of FIGS. 20 a-b,prepared substrate 162 of FIGS. 25-26, substrate 180 of FIG. 27,substrate 190 of FIGS. 28-29, substrate 210 of FIG. 30, and substrate222 of FIGS. 31-32—and those whose structured surface is notsubsequently machined, such as the final mold that is used for embossingor casting-and-curing retroreflective sheeting.

Suitable materials for use with bulk mold substrates are well known tothose of ordinary skill in the art, and generally include any materialthat can be machined cleanly without burr formation and that maintainsdimensional accuracy after groove formation. A variety of materials suchas machinable plastics or metals may be utilized. Acrylic is an exampleof a plastic material; aluminum, brass, electroless nickel, and copperare examples of useable metals.

Suitable materials for use with replicated mold substrates that are notsubsequently machined are well known to those of ordinary skill in theart, and include a variety of materials such as plastics or metals thatmaintain faithful fidelity to the prior structured surface. Thermallyembossed or cast plastics such as acrylic or polycarbonate can be used.Metals such as electrolytic nickel or nickel alloys are also suitable.

Suitable materials for use with replicated mold substrates whosestructured surface is subsequently machined are also well known to thoseof ordinary skill in the art. Such materials should have physicalproperties such as low shrinkage or expansion, low stress, and so onthat both ensure faithful fidelity to the prior structured surface andthat lend such materials to diamond machining A plastic such as acrylic(PMMA) or polycarbonate can be replicated by thermal embossing and thensubsequently diamond machined. Suitable hard or soft metals includeelectrodeposited copper, electroless nickel, aluminum, or compositesthereof.

With respect to retroreflective sheetings made directly or indirectlyfrom such molds, useful sheeting materials are preferably materials thatare dimensionally stable, durable, weatherable and readily formable intothe desired configuration. Examples of suitable materials includeacrylics, which generally have an index of refraction of about 1.5, suchas Plexiglas resin from Rohm and Haas; thermoset acrylates and epoxyacrylates, preferably radiation cured, polycarbonates, which have anindex of refraction of about 1.6; polyethylene-based ionomers (marketedunder the name ‘SURLYN’); polyesters; and cellulose acetate butyrates.Generally any optically transmissive material that is formable,typically under heat and pressure, can be used. Other suitable materialsfor forming retroreflective sheeting are disclosed in U.S. Pat. No.5,450,235 (Smith et al.). The sheeting can also include colorants, dyes,UV absorbers, or other additives as needed.

It is desirable in some circumstances to provide retroreflectivesheeting with a backing layer. A backing layer is particularly usefulfor retroreflective sheeting that reflects light according to theprinciples of total internal reflection. A suitable backing layer can bemade of any transparent or opaque material, including colored materials,that can be effectively engaged with the disclosed retroreflectivesheeting. Suitable backing materials include aluminum sheeting,galvanized steel, polymeric materials such as polymethyl methacrylates,polyesters, polyamids, polyvinyl fluorides, polycarbonates, polyvinylchlorides, polyurethanes, and a wide variety of laminates made fromthese and other materials.

The backing layer or sheet can be sealed in a grid pattern or any otherconfiguration suitable to the reflecting elements. Sealing can beaffected by use of a number of methods including ultrasonic welding,adhesives, or by heat sealing at discrete locations on the arrays ofreflecting elements (see, e.g., U.S. Pat. No. 3,924,928). Sealing isdesirable to inhibit the entry of contaminants such as soil and/ormoisture and to preserve air spaces adjacent the reflecting surfaces ofthe cube corner elements.

If added strength or toughness is required in the composite, backingsheets of polycarbonate, polybutryate or fiber-reinforced plastic can beused. Depending upon the degree of flexibility of the resultingretroreflective material, the material can be rolled or cut into stripsor other suitable designs. The retroreflective material can also bebacked with an adhesive and a release sheet to render it useful forapplication to any substrate without the added step of applying anadhesive or using other fastening means.

GLOSSARY OF SELECTED TERMS

-   An “array of neighboring cube corner elements” means a given cube    corner element together with all adjacent cube corner elements    bordering it.-   “Closed cavity” means a cavity that is substantially entirely    bounded by walls of protrusions or pyramids on all sides.-   “Compound face” means a face composed of at least two    distinguishable faces (referred to as “constituent faces”) that are    proximate each other. The constituent faces are substantially    aligned with one another, but they can be offset translationally    and/or rotationally with respect to each other by relatively small    amounts (less than about 10 degrees of arc, and preferably less than    about 1 degree of arc) to achieve desired optical effects as    described herein.-   “Cube corner cavity” means a cavity bounded at least in part by    three faces arranged as a cube corner element.-   “Cube corner element” means a set of three faces that cooperate to    retroreflect light or to otherwise direct light to a desired    location. Some or all of the three faces can be compound faces.    “Cube corner element” also includes a set of three faces that itself    does not retroreflect light or otherwise direct light to a desired    location, but that if copied (in either a positive or negative    sense) in a suitable substrate forms a set of three faces that does    retroreflect light or otherwise direct light to a desired location.-   “Cube corner pyramid” means a mass of material having at least three    side faces arranged as a cube corner element.-   “Cube height” means, with respect to a cube corner element formed on    or formable on a substrate, the maximum separation along an axis    perpendicular to the substrate between portions of the cube corner    element.-   “Dihedral edge” of a cube corner element is an edge of one of the    three faces of the cube corner element that adjoins one of the two    other faces of the same cube corner element. Note that any    particular edge on a structured surface may or may not be a dihedral    edge, depending upon which cube corner element is being considered.-   “Direct machining” refers to forming in the plane of a substrate one    or more groove side surfaces typically by drawing a cutting tool    along an axis substantially parallel to the plane of the substrate.-   “Geometric structure” means a protrusion or cavity having a    plurality of faces.-   “Groove” means a cavity elongated along a groove axis and bounded at    least in part by two opposed groove side surfaces.-   “Groove side surface” means a surface or series of surfaces capable    of being formed by drawing one or more cutting tools across a    substrate in a substantially continuous linear motion. Such motion    includes fly-cutting techniques where the cutting tool has a rotary    motion as it advances along a substantially linear path.-   “Non-machinable” means, with respect to a structured surface that    extends along a reference plane, that such structured surface cannot    be fabricated simply by drawing a cutting tool along paths    substantially parallel to the reference plane.-   “Nondihedral edge” of a cube corner element is an edge of one of the    three faces of the cube corner element that is not a dihedral edge    of such cube corner element. Note that any particular edge on a    structured surface may or may not be a nondihedral edge, depending    upon which cube corner element is being considered.-   “PG cube corner element” stands for “preferred geometry” cube corner    element, and is defined in the context of a structured surface of    cube corner elements that extends along a reference plane. For the    purposes of this application, a PG cube corner element means a cube    corner element that has at least one nondihedral edge that: (1) is    nonparallel to the reference plane; and (2) is substantially    parallel to an adjacent nondihedral edge of a neighboring cube    corner element. A cube corner element whose three reflective faces    are all rectangles (inclusive of squares) is one example of a PG    cube corner element.-   “Prepared substrate” means a substrate that has a plurality of faces    corresponding to only portions of a desired or final structured    surface.-   “Protrusion” has its broad ordinary meaning, and can comprise a    pyramid.-   “Pyramid” means a protrusion having three or more side faces that    meet at a vertex, and can include a frustum.-   “Reference plane” means a plane or other surface that approximates a    plane in the vicinity of a group of adjacent cube corner elements or    other geometric structures, the cube corner elements or geometric    structures being disposed along the plane.-   “Retroreflective” means having the characteristic that obliquely    incident incoming light is reflected in a direction antiparallel to    the incident direction, or nearly so, such that an observer at or    near the source of light can detect the reflected light.-   “Structured” when used in connection with a surface means a surface    that has a plurality of distinct faces arranged at various    orientations.-   “Symmetry axis” when used in connection with a cube corner element    refers to a vector that originates at the cube corner apex and forms    an equal acute angle with the three faces of the cube corner    element. It is also sometimes referred to as the optical axis of the    cube corner element.-   “Transition line” means a line or other elongated feature that    separates constituent faces of a compound face.

All patents and patent applications referred to herein are incorporatedby reference. Although the present invention has been described withreference to preferred embodiments, workers skilled in the art willrecognize that changes can be made in form and detail without departingfrom the spirit and scope of the invention.

What is claimed:
 1. A prepared substrate comprising an array of cubecorner cavities and protrusions interspersed between the cube cornercavities, wherein the horizontal cross section of the protrusions aretriangles.
 2. The prepared substrate of claim 1 wherein the protrusionshave steeply inclined side walls.
 3. The prepared substrate of claim 2wherein the protrusions further comprise machined faces.
 4. The preparedsubstrate of claim 3 wherein an angle in excess of 10 degrees is formedbetween the machine face and the steeply inclined side walls.
 5. Theprepared substrate of claim 4 wherein the angle ranges up to about 45degrees.
 6. A negative replication of the prepared substrate of claim 1.7. A positive replication of the prepared substrate of claim 1.